Adaptive signal processing systems, such as adaptive array antennas, may employ several thousand complex weights for steering an antenna beam (or beams). In microwave communication systems, in which a number of different elements are combined to meet the functional demands of the system, component fabrication has taken the form of monolithic microwave integrated circuits (MMICs) which offer low cost, miniaturization and high reliability. In such circuits, semiconductor material such as gallium arsenide, indium phosphide and other III-V components make it possible to integrate a large number of signal processing elements to meet the needs of the system. One type of signal processing component which has enjoyed use in complex weight networks in such systems is the dual gate field effect transistor (e.g. dual gate MESFET).
A schematic diagram of a dual gate FET and the attenuation characteristics thereof are shown in FIGS. 1 and 2, respectively. For purposes of the present description, the dual gate FET 10 is considered to be a dual gate MESFET having a pair of gate electrodes 11 and 12, and respective source (drain) and drain (source) electrodes 13 and 14. When the dual gate FET 10 is used as a microwave attenuator device an RF input signal is applied to input gate 11 while the second gate 12 is coupled to a control voltage for adjusting the attenuation provided by the MESFET at output (drain) electrode 13. The source electrode 14 is coupled to a source of reference potential (ground) as shown.
In operation, in response to a variation in a DC control voltage applied to control gate 12, the attenuation of the input signal applied to the second gate 11 varies from a minimum value represented by minimum attenuation vector 21, shown in FIG. 2, to a maximum value represented by maximum attenuation vector 22. Here, the degree of attenuation is assumed to be greater than 15 dB and there is a substantial phase shift .phi.1 between the minimum attenuation vector 21 and the maximum attenuation vector 22, caused primarily by the lowering of the transconductance of the MESFET as the depletion region beneath the second gate is widened with the increase in control voltage. (As pinch-off is approached, other parameters (e.g. intrinsic resistance, capacitance) are varied in a complex interaction to produce a net effect of a rotation of the attenuation vector through phase shift angle .phi.1.) As a result, conventional dual gate MESFET attenuators are not practically useful in complex weighting devices where a substantial amount of attenuation (on the order of 40 dB) is required with minimal phase shift over that attenuation range.